When the question/response is "what" but not just "what", implying that the speaker is responding in an irritated or frustrated or confused way, and that an additional "...is the problem" or "...is your issue" or "...the hell is going on" comes right after but is unsaid.

If the person wants, they can say the unsaid, for emphasis.

Examples:

Girl: (for the 10th time) "Um can you seriously come over here and look at this?"

FriendofGirl: (busy doing something else) "Whaz?!?!! What IS it? What is the deal with you, biznitch?!!"

or

NewsReporter: "...coming down the I-95 at 87 miles an hour, then flipping over and bursting into flame near the exit to Main street!"

Person watching TV: (disbelief) "Whaaz...? I was just over there 5 minutes ago!!"